Leakage cancellation circuits

ABSTRACT

Technologies for RFID positioning and tracking apparatus and methods are disclosed herein. The apparatus and methods disclose a radio-frequency identification positioning system that includes a radio-frequency identification reader and a phased-array antenna coupled to the radio-frequency identification reader. Techniques are applied to reduce in-reader and in-antenna signal leakages. Techniques are applied to position and track RFID tags. Circuits with leakage cancellation abilities are also disclosed. Reflective vector attenuators with tunable impedance load are also disclosed. Polarization adjustable antennas with matching circuits used in the RFID positioning system are also disclosed. Circuits to re-transmit a received signal at a higher amplitude to enhance radio link range are also disclosed. Techniques are applied to increase the level of scattered radio signals from RFID tags.

PRIORITY CLAIM

This application is a non-provisional utility patent application that isa divisional application from a pending non-provisional utility patentapplication (application Ser. No. 14/516,585) that claims the benefit ofU.S. Provisional Patent Application No. 61/893,217, which was filed onOct. 19, 2013, all of which are incorporated by reference herein in itsentirety.

FIELD OF THE INVENTION

At least one embodiment of the present invention pertains toradio-frequency identification systems to position and track stationaryand moving objects. At least one embodiment of the present inventionfurther pertains to signal leakage canceling in radio-frequencyidentification system.

BACKGROUND

The subject matter discussed in the background section should not beassumed to be prior art merely as a result of its mention in thebackground section. Similarly, a problem mentioned in the backgroundsection or associated with the subject matter of the background sectionshould not be assumed to have been previously recognized in the priorart. The subject matter in the background section merely representsdifferent approaches, which in and of themselves may also be inventions.

Radio-frequency (RF) identification (RFID) is the wireless use ofelectromagnetic fields to transfer data, for the purposes ofautomatically identifying, detecting, positioning and tracking tags.

In some cases, RFID systems comprise RFID tags and readers. RF readerscomprise an antenna and electronic power circuitry to locate nearby tagsand receive encoded information using radio frequency communication. Thereader emits high frequency radio waves that any nearby passive RF tag,which absorbs radio energy to power-up its own integrated circuit orload-modulate the radio energy to beam back an ID number to the reader.The reader can also write basic information to the chip on the tag. Forexample, if the RF tag is inside a book, a code written to the chip mayindicate the book has been checked out. A security gate reader will thenreceive this information from the tag to allow the book to pass through.

Many models of reader are hand-held devices (resembling a barcodescanner or pricing gun), but they can also be fixed in place (such as insecurity gates or counter-tops) or even hidden completely (embedded inceilings or walls).

A typical RFID tag consists of a microchip attached to a radio antennamounted on a substrate. The microchip can store information such asvehicle license plate or payment related information. RFID tags, whichuse radio waves to communicate their identity and other information tonearby readers, can be passive or active. Passive RFID tags are poweredby the reader and do not have a battery. Active RFID tags are powered bybatteries and able to generate own radio signal.

RFID tags can store a range of information from one serial number toseveral pages of data. Readers can be mobile so that they can be carriedby hand, or they can be mounted on a post or overhead. Reader systemscan also be built into the architecture of a cabinet, room, or building.

These radio-frequency identification tags can be attached to objectssuch as vehicles, persons and other movable objects. The radio-frequencytags contain electronically stored information and may be accessed orprogrammed wirelessly by RFID readers in a distance.

SUMMARY

Techniques introduced here provide a feed-forward and open-loop leakagecancellation circuit which includes a quadrature down-mixer configuredto down-mix a first copy of a receiver signal with a first copy of atransmitter signal and generate two outputs. The circuit also includes afirst low pass filter coupled to the in-phase output of the down-mixerand the first low pass filter is configured to filter the output andgenerate a first reference voltage. The circuit includes a second lowpass filter coupled to the quadrature output of the down-mixer and thesecond low pass filter is configured to filter the output and generate asecond reference voltage. The circuit also includes a linear combinercoupled to the first low pass filter and the second low pass filter. Thelinear combiner is configured to generate a first control voltage and asecond control voltage. The first control voltage is related to a firstlinear combination of the first reference voltage, the second referencevoltage and a first coefficient. The second control voltage is relatedto a second linear combination of the first reference voltage, thesecond reference voltage and a second coefficient. The circuit includesa quadrature up-mixer coupled to the linear combiner. The quadratureup-mixer is configured to up-mix the first control voltage, the secondcontrol voltage and a second copy of the transmitter signal. Thequadrature up-mixer is configured to add output of the up-mixer back toreceiving signal and cancel transmitter leakage.

Techniques introduced here also provide a radio-frequency identificationpositioning system that includes a radio-frequency identification readerand a phased-array antenna coupled to the radio-frequency identificationreader. The phased-array antenna is configured to contain at least tworadiating elements and capable of obtaining angle of arrival informationand location information of at least two radio-frequency identificationtags. The at least two radio-frequency identification tags areconfigured to store the location information. Each of the at least tworadiating elements is driven by a dedicated radio frequency up-converterfor transmission and each of the at least two radiating elements isdriven by a dedicated radio frequency down-converter for reception.Phase delays between the at least two radiating elements are configuredto create radiation peaks at different azimuth or elevation angles toestimate direction of the at least two radio-frequency identificationtags and identify the location of the reader.

Techniques introduced here also provide a reflective vector attenuatorthat includes a hybrid coupler that comprises a first 3 dB port and asecond 3 dB port. The attenuator also includes a first terminationcircuit coupled to the first 3 dB port and the first termination circuitis configured to have a first adjustable termination impedance. Theattenuator also includes a second termination circuit coupled to thesecond 3 dB port and the second termination circuit is configured tohave either a second adjustable termination impedance or a fixedimpedance.

Techniques introduced here provided a feed-back and closed-loop leakagecancellation circuits which includes a quadrature down-mixer configuredto mix a receiver signal with either a copy of a transmitter signal or alocal oscillator signal and generate two outputs. The circuit alsoincludes a first low pass filter coupled to the quadrature down-mixerand the first low pass filter is configured to filter the one output ofthe mixer and generate a first reference voltage. The circuit includes asecond low pass filter coupled to the quadrature down-mixer and thesecond low pass filter is configured to filter the one output of themixer and generate a second reference voltage. The circuit also includesa linear combiner coupled to the first low pass filter and the secondlow pass filter. The linear combiner is configured to generate a firstcontrol voltage and a second control voltage. One configuration of thelinear combiner includes two analog to digital converters to digitizethe reference voltages; a digital signal processor unit to linearlyprocess the converted voltage; and two digital to analog converters toproduce two control signals. Another configuration of the linearcombiner includes four analog multipliers and two analog signalcombiners to produce two control signals. The circuit includes a vectorattenuator coupled to the linear combiner. The vector attenuator iscontrolled by the two control signals to produce a negative copy of thetransmission leakage. The produced negative leakage is combined back tothe receiving signal before the quadrature mixer to cancel thetransmission leakage into the mixer. An amplifier may be insertedbetween the leakage combiner and quadrature down-mixer to reduce thenoise figure of the down-converter. Two high pass filters or bandpassfilters are individually coupled to the in-phase and quadrature outputsof the down-mixer to further reduce transmitter leakage from thedown-converted receiving signals for demodulation.

Techniques introduced here also provide a radio frequency identificationtag that comprises a receiving antenna and the receiving antenna isconfigured to receive external signals. The radio frequencyidentification tag also includes an amplifier coupled to the receivingantenna and the amplifier is configured to amplify the external signalsand generate amplified signals. The tag also includes an envelopedetector coupled to the amplifier, wherein the envelop detector isconfigured to demodulate the amplified signals from the amplifier andgenerate demodulated signals. The tag also includes a processor coupledto the envelop detector and the processor is configured to process thedemodulated signals to initiate activities including but not limiting toreading or writing memory, generate processed signals, interface with aninput or output device, or communicate with a remote host. The tag alsoincludes a single-pole-single-throw switch coupled to the amplifier andthe processor and the switch is controlled by the processor to amplitudemodulate the amplified signal to generate modulated messages. The tagalso includes a transmitting antenna coupled to the switch and thetransmitting antenna is configured to broadcast the modulated messages.The tag also includes a memory unit coupled to the processor and thememory unit is configured to store information. One configuration of thetag contains a vector attenuator coupled to the amplifier and theretransmitted signal may be phase or frequency modulated by the vectorattenuator.

Techniques include here also provide a radio relay device that includesa receiving antenna, an amplifier coupled to the receiving antenna, anda transmitting antenna to re-transmit the received signal at a higheramplitude. The relay device further includes a envelop detector coupledto the amplifier, to compare the output power level to a pre-determinedlevel to identify the operating condition of the amplifier. The gain orphase of the amplifier may be further tuned to either suppressoscillation or to keep the amplifier operating linearly.

Other aspects of the technology introduced here will be apparent fromthe accompanying figures and from the detailed description whichfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and characteristics of the presentinvention will become more apparent to those skilled in the art from astudy of the following detailed description in conjunction with theappended claims and drawings, all of which form a part of thisspecification. In the drawings:

FIG. 1 illustrates a diagram of a radio-frequency identification (RFID)positioning system.

FIG. 2A illustrates a diagram of RFID reader structures with in-readerand in-antenna transmission leakage cancellers.

FIG. 2B illustrates a diagram of a feed-forward transmission leakagecanceller.

FIG. 3A illustrates a diagram of a reflective vector attenuator withswitchable LRC terminations.

FIG. 3B illustrates a diagram of a reflective vector attenuator withshunt-switch-loaded transmission lines.

FIG. 3C illustrates a diagram of a reflective vector attenuator withseries and shunt tunable LRC terminations.

FIG. 3D illustrates a diagram of a reflective vector attenuator with adouble variable-resistor-loaded transmission line.

FIG. 3E illustrates a diagram of a reflective vector attenuator withphase-delayed variable resistors.

FIG. 3F illustrates a diagram of a phase delay circuit to replace ⅛wavelength transmission line

FIG. 3G illustrates a diagram of a circuit to replace ⅛ wavelengthtransmission line with negative delay.

FIG. 4A illustrates a diagram of a dual adjustable polarized antennawith controllable polarization correlations.

FIG. 4B illustrates a diagram of a polarization adjustable antenna withmatching circuits.

FIG. 4C illustrates a diagram of a stubbed matching circuit for a 25/50ohm variable load.

FIG. 4D illustrates a diagram of a branch selective matching circuit fora 25/50 ohm variable load.

FIG. 5A illustrates a diagram of a bi-static RFID tag with amplifier andswitch modulator.

FIG. 5B illustrates a diagram of a bi-static RFID tag with amplifiermodulator and vector attenuator for stability control and modulationdepth enhancement

FIG. 5C illustrates a diagram of a bi-static RFID tag with polarizationcontrol.

FIG. 5D illustrates a diagram of an amplifier with adjustable gain andphase.

DETAILED DESCRIPTION

References in this specification to “an embodiment,” “one embodiment,”or the like, mean that the particular feature, structure, orcharacteristic being described is included in at least one embodiment ofthe present invention. Occurrences of such phrases in this specificationdo not all necessarily refer to the same embodiment, however.

In the modern world, RFID tags and readers are used in the daily lifefor access control and asset management. However, utilization of RFIDsystems including RFID reader and tags are limited. Some limitingfactors include the multi-path wave propagation effect, limitedactivation power threshold due to transistor threshold, and less thanunity of back-scattering modulation efficiency for existing passive andsemi-passive RFID tags. Back-scattering modulated RFID tags required thereader to operating in the full-duplex mode, which is simultaneousoperation of the transmitter and receiver. The RFID reader receiver hasa variable noise figure subject to potential de-sensitization by itstransmission leakage. A breakthrough in reader sensitivity and tagmodulation efficiency would greatly increases read distances of RFIDtags, and opens the door to a whole host of applications. Applicationsthat were not realizable at the moment would be achievable with doubledor quadrupled read distances.

Most RFID tags may be divided into two parts. A first part includes anintegrated circuit for storing and processing information, modulatingand demodulating a radio-frequency (RF) signal, and other specializedfunctions. A second part includes an antenna for receiving the signal.Passive RFID tag uses one antenna for transmit and receiving, at thesame time, and the back-scattering modulation is performed by changingthe load impedance of the antenna. The back-scattered signal is only aportion of the incoming signal, and some energy is used to power-up theinternal circuits of passive RFID tags. An amplifier and a circulatormay be added to a single-antenna back-scattering tag to increasemodulation efficiency. Such approach has a high risk of oscillation dueto antenna impedance variation. A loop is formed by the amplifier andcirculator lines. The change of antenna impedance and return loss isdirectly contributed to the loop gain, and the loop amplifier willoscillate if the loop gain is one or higher for this ring. It will beappreciated to have a more reliable way to utilize amplifier in RFIDtags to enhance the modulation efficiency.

There are three general types of RFID tags: active RFID tags, whichcontain a battery and can transmit signals autonomously; passive RFIDtags, which have no battery and require an external wave source topower-up the tag circuits and to be provoked for tag signaltransmission; and battery assisted passive (BAP) or semi-active tagswhich require an external source to power-up the tag circuit butexternal wave source as the RF source is still required to be loadmodulated for tag signal transmission. In general, battery assistedpassive tags have lower RF triggering power, and a longer reader range.However, the longer read range required better RFID reader sensitivitydue to higher propagation loss.

An RFID reader/receiver can transmit RF waves to nearby RFID tags forinformation exchange. Tag read range is determined by the readertransmission power, propagation loss, polarization match between thereader and tag, and tag activation power threshold. In a realenvironment there are multiple propagation paths exist between thereader and tag. As the tag moves around, the combined wave from themultiple paths would leads to variable field strength, and the combinedwave polarization axial ratio and orientation will change. A strongfading effect will be observed on fixed transmitting and receivingantenna polarizations. The reader need to have sensitivity lower thanthe back-scattered tag response, which could be very low for long rangetags even in free space enviroment. The reader sensitivity is determinedby its noise figure and dynamic range, and level of de-sensing bytransmission leakage. The tag activation distance may be achieved byincreasing the transmission power or decreasing the tag activationpower. The first method leads to a stronger de-sensing transmissionleakage, and the second method leads to more path-loss. Either methodcould benefit from a reader with less leakage.

In full-duplex of RFID systems the leakage of transmission signal mayoccur in the reader, between the transmitting and receiving antenna, orbounced off the surrounding environment. The in-reader leakage isintroduced by internal PCB coupling, power coupling, wave resonance,device leakage, or proximity coupling. The antenna-introduced couplingwould be return-loss dependent in mono-static setup, or mutual couplingdependent in bi-static setup. Mono-static means that one antenna isresponsible for simultaneous transmission and receiving, and bi-staticmeans that two antennas are individually responsible for transmissionand receiving. A circulator or directional coupler is commonly used toduplex the transmission and reception in a mono-static radio.Additionally, environmental change such as a vehicle or human beingapproaching or departure from the antenna would also introducetransmission leakage. These three types of leakage require multiplesolutions, and the antenna introduced leakages require much fastertracking for cancellation. An existing leakage canceller approachutilize a combination of variable attenuator and phase shifter tomodulate a copy of transmission signal to produce a negative copy oftransmission leakage. However, the existing reflective phase shiftercould not provide a continuous tuning adjustment once over 360 degreesto track leakages without resetting, which increase processingcomplexity and limit the speed of tuning. Also the attenuator may have afixed set of tuning range, thus limit the tuning resolutions.

The inventions described below are a comprehensive approach to increaseRFID reading distance by reducing reader transmission leakage andreceiving noise figure, improving polarization matching between readerand tag antennas, and increase tag back-scattering modulationefficiency. With long range RFID reader and tags, a new apparatus ofapplying the RFID readers and tags for electronic positioning andguidance purpose is to be disclosed below. The new positioning systemhas a much shorter propagation delay and processing time than satellitepositioning system. It is also suitable for indoor or outdoorenvironment.

FIG. 1 illustrates a diagram of a radio-frequency identification (RFID)positioning system 100. In the radio-frequency identification (RFID)positioning system 100, a RFID reader 105 with a direction-findingphased-array antenna is used to read a first RFID tag 135, a second RFtag 140 and a third RF tag 145 in its interrogation zone. The angle ofarrival information of each tag is obtained using the electronicallycontrolled phased array. Three RFID tags, whose coordinates are storedin their memory, are used as position markers for the RFID reader 105.The RFID reader 105 shall interrogate these tags to obtain theircoordinates and subsequently calculated the reader's own coordinates andorientation.

In one embodiment of the techniques provided, the RF reader 105 couldcouple to a first directional antenna 120, a second directional antenna125 and a third directional antenna 130. The RF reader 105 could couplethe first directional antenna 120 through transmitting (TX) signal 111and receiving signal (RX) 112. In another embodiment, the RF reader 105could couple the second directional antenna 125 through TX signal 113and receiving signal (RX) 114. In another embodiment, the RF reader 105could couple the third directional antenna 130 through transmitting (TX)signal 115 and receiving signal (RX) 116.

In one embodiment, the phased-array antenna in the RF reader 105contains at least two radiating elements, and each radiating element isdriven by a dedicated radio frequency (RF) up-converter for transmissionand RF down-converter for reception. The phase delays between adjacentradiating elements are electronically controlled to create radiationpeaks at different azimuth or elevation angles to estimate the directionof tag being interrogated.

The reader 105 position and orientation can be determined using thetriangulation principle. Two out of three RFID tags (135, 140 and 145)are sufficient for the reader 105 to determine its longitude, latitudeand orientation for a reader with a 1-D phased-array, if the reader andthree tags (135, 140 and 145) are not located on a straight line. Also a2-D phased-array with azimuth and elevation beam control is necessaryfor the extraction of the 3-D reader coordinates and orientation. Thetrack of reader 105 may be obtained by continuously monitoring itsposition, and its moving speed may be calculated accordingly.

The RFID tags (135, 140 and 145) may store additional information suchas speed limit and road condition. This information may be furtherprocess by the processor or host of the RFID reader for guidancepurpose. In one embodiment, a wired programmer 150 may be directlyconnected to the tag to update its memory on demand. In anotherembodiment, the RFID tags may also be wirelessly programmed by awireless programmer 160. Real-time update of these tags can be achievedusing Internet-connected programmers, such as the wireless programmer160.

In one embodiment, relative location information of RFID tag 140, RFIDtag 135, RFID tag 145, directional antenna 125 and directional antenna130 can be determined. In one embodiment, distance 177 between RFID tag135 and RFID tag 140 can be determined. In one embodiment, distance 178between RFID tag 135 and RFID tag 145 can be determined. In oneembodiment, distance 179 between RFID tag 145 and RFID tag 140 can bedetermined. In one embodiment, distance 174 between RFID tag 135 anddirectional antenna 125 can be determined. In one embodiment, distance175 between RFID tag 140 and directional antenna 125 can be determined.In one embodiment, distance 176 between RFID tag 145 and directionalantenna 125 can be determined. In one embodiment, angle 171 between RFIDtag 135 and directional antenna 125 can be determined. In oneembodiment, angle 172 between RFID tag 140 and directional antenna 125can be determined. In one embodiment, angle 173 between RFID tag 145 anddirectional antenna 125 can be determined.

FIG. 2A illustrates a diagram of RFID reader structures with in-readerand in-antenna transmission leakage cancellers. In one embodiment, aRFID reader 200 comprises a digital processor 201 that includes memory202. The digital processor 201 can pass information to a host controller207, a key pad 208 and a display 209.

In one embodiment, digital processor 201 pass signals to DAC(Digital-to-Analog Converter) 203. The DAC 203 can output baseband (BB)transmitting signals 205. Receiving signals 206 can be passed into ADC(Analog-to-Digital-Converter) 204 and then the output of the ADC 204 canbe passed into digital process 201 for demodulation and decoding. Thesebaseband TX signals are used to modulate the local oscillator (LO)reference signal to generate the RF TX signal. In one embodiment, theRFID reader 200 contains a phase lock loop (PLL) oscillator 213 for LOsignal generation. In a phased array, the local oscillator signals areshared among all radiating elements. The RF receiving signal isamplified by a low noise amplifer 224, and down-converted by thequadrature down-mixer 212 to generate baseband RX signals 217. In someembodiments, the baseband RX BB signals are same signals as 206. Amemory circuit 202 is connected to digital processor for the storage ofID contents, control program, and configuration settings. Localoperation of the reader without a host is enabled by a keypad 208 and adisplay 209, both are connected to the digital processor 201. Remoteprogramming of the reader and tags are also supported through a wired orwireless connection from a host controller 207.

In one embodiment, a RFID reader 200 also comprises a quadrature (Quad)Up-Mixer 211 and the quad up-mixer 211 can up-convert the transmittingbaseline signal 216.

In one embodiment, a vector modulator 214, a RF power detector 215 areused to mitigate in-reader coupling 218 when are possibility of leakagefrom TX line to RX line. The RF power detector 231 is placed after thesummating directional couplers 222 and 223 on the RX path inside theantenna. The output of this RF detector is the error indicator for thecalibration of vector attenuator 214.

In one embodiment, a RFID reader 200 is a bi-static reader that isdriven by antennas for Bi-static operation 230 that includes tworadiating antenna elements: a transmitting (TX) antenna 250 fortransmitting the reader to tag (R2T) interrogation signals and areceiving antenna 255 for receiving the tag to reader (T2R) responsesignals.

In the bi-static antenna 230, a portion of transmitting (TX) signal isexpected to be leaked from the transmit antenna 250 to the receiving(RX) antenna 255 due to near field coupling 251. Farfield reflection ofthe transmitted signal from the objects near the antennas is anothersource of leakage.

In one embodiment, inside the RFID reader the proximity of RFtransmitting components and transmission lines may also lead to TXcoupling to the RF receiving components and transmission lines. Althoughthe coupling due to the receiver and antenna hardware are relativelystable, the near-field coupling and farfield reflections from theobjects are neither stable nor predictable. Some existing may use anin-reader canceller to reduce the leakage for the RX mixer. Compensatingfor the leakage near the antenna would provide a much wider cancellationbandwidth since the phase delay of the leakage and cancellation circuitsare reduced. To correct the TX to RX leakage, a portion of TX signal isintentionally introduced to the RX path using two directional couplers234, 233 and a vector modulator 240 inside the bi-static antenna 230.The name of vector modulator is interchangeable with quadrature up-mixerhere. The vector attenuator 240 is adjusted to the value so that theintentionally introduced TX leakage cancels the unintentional TX to RXleakage. An RF power detector 231 is placed after the summatingdirectional coupler on the RX path inside the antenna. The output ofthis RF detector is the error indicator for the adjustment of vectormodulator 214. The circuits to produce the control signals are shown inFIG. 2B.

In another embodiment, an amplifier is used to amplify the signals in RFband to reduce the noise figure of reader. The RX signal to be amplifiedis first cleaned by the in-reader leakage canceller, which contains twodirectional couplers, one vector modulator, and one RF power detector.This function of this canceller is to provide to the quadraturedown-mixer a low leakage RF signal so that the noise figure of receiveris least. The down-mixer is a non-linear device with lower third orderinterception (IP3) than passive components. The low noise amplifier isalso subject to its own IP3 limit.

In one embodiment, a mono-static antenna 270 is used to transmit R2T andreceive T2R signals in the mono-static type of RFID reader. The antenna270 contains a antenna radiator 263. The antenna radiator 263 transmitsand receives signals simultaneously. An antenna duplexer 262 isconnected to the antenna radiator. The duplexer 262 has an input portfor transmitting and an output port for receiving. In this invention theantenna duplexer 262 is integrated with the antenna, and is preferred tobe placed as closed to the antenna radiator 263 as possible. Once again,two couplers 268 and 269, one power detector 260 and one vectormodulator 261 are used for leakage cancellations inside the antenna. Thetwo couplers 268 and 269 are preferred to be placed as close to duplexer262 as possible. A cable 230 is used to connect the reader TX outputport to the antenna TX input port 264; and another cable 221 is used toconnect the antenna RX output port 265 to the reader RX input port.Unlike other RFID reader system configurations, the arrangement of thecanceller and cable in this invention minimize the delay of theantenna-induced leakage. The phase difference between the leakage pathand cancellation path is also minimized, thus leads to optimizedcancellation bandwidth and depth. The transmitter leakage comes in twoways: one is from the internal circulator, and the other is from theantenna reflection. If the two leakage path may not be minimized, theirdifference should be minimized. This delay balance is the keycontributor of the cancellation bandwidth and depth for both types ofleakages.

In one embodiment, the benefits of the distributed leakage cancellation,i.e. using an antenna integrated canceller and a reader integratedcanceller, are better receiving noise figure and higher bandwidth. Thecontribution of this invention is to achieve both benefits at the sametime, even for RFID reader systems with antenna cables or antennaswitches. A single canceller design could be only be delay matched toeither the in-reader leakage or the antenna-induced leakge if antennacables are used. The cable will produce a frequency dependent phasedelay. In multi-antenna RFID reader configuration, the cables could betens of wavelengths long. A phase error of 10 degree would limit theleakage depth to about 15 dB. Static control of the vector modulator isonly able to cancel static transmission leakage. Because transmissionleakages changes over time due to equipment temperature variation, cablemovement, or environment change, dynamic control of leakage canceller isemployed in this invention.

FIG. 2B illustrates a diagram of a feed-forward transmission leakagecanceller. In one embodiment, a feed-forward transmissionleakage-cancellation circuit 290 may be employed in the antenna or inthe reader to address dynamic transmission leakage.

In one embodiment, the feed-forward transmission leakage-cancellationcircuit 290 contains five directional couplers. A first coupler 291 anda second coupler 292 are on the TX path 297. A third coupler 293, afourth coupler 294 and a fifth coupler 295 are on the RX path. Thecoupler 293 is optional.

In one embodiment, a copy of TX signal 297 is down-mixed with a copy ofRX signal 299 in a quadrature down-mixer. The DC-containing outputs ofthis mixer are low pass filtered by a first low pass filter 283 and asecond low pass filter 284 to produce the reference voltages, EVI 285and EVQ 286. A linear combiner circuit 287 is used to produce twocontrol voltages, CVI 288 and CVQ 289, to drive a quadrature up-mixer281, whose LO references are taken from the TX path using anotherdirectional coupler 294. An optional RF power detector 272 monitors thequality of this canceller during the calibration stage.

In one embodiment, the function of the liner combiner 287 is to productthe control voltages CVI 288 , CVQ 289 according to the followingequation: CVI=a*EVI+b*EVQ+c, CVQ=d*EVI+e*EVQ+f, Where a, b, c, and d arecoefficients to address the dynamic leakages, and c and f arecoefficients to address stationary leakages. One configuration of thelinear combiner includes four analog multipliers and two analog signalcombiners to produce two control signals. Another configuration of thelinear combiner includes two analog to digital converters to digitizethe reference voltages; a digital signal processor unit to linearlyprocess the converted voltage; and two digital to analog converters toproduce two control signals.

In one embodiment, A stationary environment produces stationary controlsignals, and a varying environment produces a varying control signals.The frequency of the control signal is determined by the relative movingspeed of antenna versus the environment. As the environment varies, thetwo error voltages 285 and 286 also vary. The required control voltages288 and 289 are automatically generated and self-aligned. The uniquebenefits of this dynamic canceller are that it is able to cancel theleakage introduced by moving objects without the adjustments of thecoefficients of the linear combiner 287. The feed-forward canceller 290is an open-loop controlled system, therefore a stable control system.The environment tracking speed is limited by the bandwidth of LPF 283and 284. The bandwidth is only limited by the tag signal bandwidth. Incomparison, a feedback-controlled system is potentially unstable,especially at high loop gain. The bandwidth of a feedback-controlledsystem is additionally affected by the loop gain to avoid looposcillation.

A slowing varying leakage canceller may be used to cancel in-reader orin-antenna transmission leakages due to temperature changes. By limitingthe loop bandwidth, a high cancellation depth is achievable at high loopgain. Such configuration requires highly accurate and monotonicallyadjustable vector modulators. A few low-cost and simple-structuredpassive reflective vector modulators are presented below, all of whichare contains a 3 dB hybrid coupler and terminations at both hybridports. FIG. 3A illustrates a diagram of a reflective vector attenuatorwith switchable LRC terminations. A reflective vector attenuator 300,comprises a first input/ouput port 304, a second input/output port 301,a 3 dB hybrid 317 which has a 3 dB port a and a 3 dB port b. The twoports are terminated at least by a first switch-controlled resistor 311,a first switch-controlled capacitor 309, a first switch-controlledinductor 310, a first short circuit termination 312, a secondswitch-controlled resistor 315, a second switch-controlled capacitor313, a second switch-controlled inductor 314 and a second short circuittermination 316. The intended termination impedances at each port is 0,j50, −j0.02, 50 or infinity ohms. The transmission coefficient T 305 andthe reflection coefficient Γ 306 of the RVA are defined as follows:

${T = {{- \frac{j}{2}}\left( {\Gamma_{a} + \Gamma_{b}} \right)}},{\Gamma = {\frac{1}{2}\left( {\Gamma_{a} + {e^{2j^{*}c}\Gamma_{b}}} \right)}},$

where Γa 307 and Γb 308 are the reflection coefficients of the twoterminations at the hybrid ports, and c is the hybrid phase difference.Therefore, the magnitude and phase of the transmission coefficient ofthis RVA can be adjusted by the independent control of two terminationimpedances. The reflective vector attenuator in FIG. 3A allows fordiscrete adjustment of their transmission and reflection coefficients.

FIG. 3B illustrates a diagram of a reflective vector attenuator withshunt-switch-loaded transmission lines. In one embodiment, a reflectivevector attenuator 319, comprises a 3 dB hybrid 329, a first distributedtermination and a second distributed termination. The first distributedtermination comprises a first quarter-wavelength transmission line, afirst 50-ohm end termination 322, and multiple switch-enabled shortcircuit shunt terminations 323,324 and 325. The second distributedtermination comprises a second quarter-wavelength transmission line, asecond 50-ohm end termination 321, and multiple switch-enabled shortcircuit shunt terminations 326, 327 and 328. The equivalent terminationimpedance for each port is 0, 50, j50, and infinity ohms. The reflectivevector attenuator in FIG. 3B allows for discrete adjustment of theirtransmission and reflection coefficients.

FIG. 3C illustrates a diagram of a reflective vector attenuator (RVA)with series and shunt tunable LRC terminations. In one embodiment, areflective vector attenuator 339 comprises a first tunable LRCterminator and a second tunable LRC terminator. The first tunable LRCterminator contains a tunable resistor 331, a tunable capacitor 333, andan inductor 332. The second tunable LRC terminator contains a tunableresistor 336, a tunable capacitor 334, and an inductor 335. Maximumreflection tuning range may be obtained if the inductor and capacitorvalues are selected to resonant. The first tunable LRC terminator is inseries configuration and the second tunable LRC terminator is inparallel configuration. At the resonant frequency the terminatorimpedance is equal to the resistance. The adjustments of capacitorsintroduce orthogonal movements of the reflection coefficients on thesmith chart for the different terminators. Therefore, this RVA providescontinuous phase and magnitude adjustments to its transmission andreflection coefficients. It is also possible to use only two tunablecomponents to produce a small tuning-range vector attenuator.

FIG. 3D illustrates a diagram of a reflective vector attenuator (RVA)with a double variable-resistor-loaded transmission line. In oneembodiment, a reflective vector attenuator 349, comprises a fixedtermination and an adjustable termination. The adjustable terminationcomprises two adjustable resistors 341 and 343 separated by a one-eighthwavelength transmission line 345. Both tunable resistor are in shuntconnection to the shared ground 342 and 344. The fixed terminationcomprises a fixed impedance 346 at any value connected shunt to ground347. The tuning of the resistors 341 and 343 induces orthogonaladjustment of the RF transmission coefficient for the vector attenuator.Continuous and monotonic adjustment of the transmission coefficient canbe achieved by using continuously and monotonically adjustableresistors. This RVA requires only two tunable resistors for theadjustments of the transmission and reflection coefficients for thevector attenuator.

FIG. 3E illustrates a diagram of a reflective vector attenuator (RVA)with phase-delayed variable resistors. In one embodiment, a reflectivevector attenuator 319 comprises two tunable terminations. The firstadjustable terminator comprises a field effect transistor (FET) 354coupled to a terminal 355. The field effective transistor 354 acts as atunable resistor. The second adjustable terminator comprises aone-eighth wavelength transmission line 351 loaded by another FET 352coupled to another terminal 353. The FET 352 also acts as a tunableresistor. This RVA has a full magnitude and phase tuning range. In oneembodiment, the first FET 354 and the one-eighth wavelength transmissionline 351 are coupled to a 3 dB hybrid 350.

The choice of tunable resistors for any RVA includes, but not limitedto, FET, p-i-n diode, or resistor networks controlled by RF switches.The choice of tunable capacitor includes, but not limited to, varactoror capacitor networks controlled by RF switches. Any of these analogtunable components may be adjusted digitally by the controllingprocessor with the help of DAC.

FIG. 3F illustrates a diagram of a phase delay circuit to replace ⅛wavelength transmission line. In one embodiment, a phase delay circuit360 comprises a series inductor 361 and a shunt capacitor 362. One endof the capacitor 362 is connected to the inductor 361, and the other endis connected to ground 363. RF signals can pass back and forth from oneend of the inductor 361 to another end of the inductor 361.

FIG. 3G illustrates a diagram of a circuit to replace ⅛ wavelengthtransmission line with negative delay. In one embodiment, a phase delaycircuit 370 comprises a series capacitor 371 and a shunt inductor 372.One end of the inductor 372 is connected to the capacitor 371, and theother end is connected to ground 373. Likewise, RF signals can pass backand forth from one end of the capacitor 371 to another end of thecapacitor 371. In one embodiment, a phase delay circuit 370 comprises acapacitor 371, an inductor 372 coupled to both the capacitor 371 and aterminal 373. The transmitting signal can pass from one end of thecapacitor 371 to another end of the capacitor 371.

The benefits of the reflective vectors attenuators shown from FIG. 3A toFIG. 3G which is made of a 90 degree hybrid, two differentialmultipliers, and an in-phase power combiner, are the cost saving, powersaving compared to a traditional vector amplifier. Another common vectorattenuator structure includes a 360 degree phase shifter and a threeresistor attenuator. The benefits of the reflective vectors attenuatorsin FIG. 3A through FIG. 3G over the phase shifter and attenuatorapproach are the cost saving and reliability improvement. The simplestructure of the proposed reflective vector attenuators shall havebetter reliability due to the low counts of passive components andabsence of active amplifiers.

The RF transfer between a radio transmitter and a radio receiver dependsheavily on the polarization match of their antennas. Polarizationadjustment on either the reader or tag side may be used to enhance orreduce the radio power transfer in the radio link. FIG. 4A illustrates adiagram of a dual adjustable polarized antenna with controllablepolarization correlations. In one embodiment, a dual adjustablepolarized antenna 400 comprises a 3 dB hybrid 430, a first RVA 405, asecond RVA 410, and a radiating structure 401. The radiating structure401 comprises a first cross-polarized antenna radiating element 402 anda second cross-polarized antenna radiating element 403.

The choice of cross-polarized radiating elements includes, but notlimited to, two dipoles or two monopoles in orthogonal placements; acircular, a square, an annular, a “+” shaped or any other symmetricalprinted patches over a ground plane with orthogonal excitations. Theexcited wave polarizations from the first cross-polarized antennaradiating element 402 and the second cross-polarized antenna radiatingelement 403 are orthogonal. Here one signal polarization is denoted asthe CO-POL and the other is denoted as X-POL. The two RVA are used tophase shift or attenuate the CO-POL and X-POL signals independently. The3 dB hybrid 430, together with the two RVA, transforms the CO-POL andX-POL antenna inputs 420 and 425 into two new signals. The new signalsexhibit new polarization forms that are different from the intrinsicpolarizations of the radiating elements. For example, a dual circularpolarized reception may be formed using dual linear polarized radiatingelements. In this case, the axial ratios of the new polarizations aredetermined by the RVA phase shifts and the starting polarizations. The 3dB hybrid 430 may be a 90 degree hybrid when quadrature combinations ofantenna signals are desired, or a 180 degree hybrid when inphase andout-of-phase combinations of antenna signals are desired.

The attenuation ratio of the two RVA may be adjusted to affect thepolarization correlation of the two new virtual polarization forms. Forexample, when the X-POL RVA is set to total absorption mode and theCO-POL RVA is set to phase shifting mode, both antenna input ports willhave the CO-POL polarization, and their polarization correlation is100%. When the CO-POL RVA is set to total absorption mode and the X-POLRVA is set to phase shifting mode, both antenna input ports will haveX-POL polarization with 100% polarization correlation. When both RVA areset to phase shifting mode and their attenuation are equal, the two newoutput polarizations will be orthogonal and their polarizationcorrelation is 0. When this adjustable polarization antenna is used witha RFID reader, the polarizations of the antennas may be changed ondemand based on the number of antennas on each tag. RFID tags with oneantenna are best interrogated with a pair of polarization correlatedantennas, and RFID tags with dual polarized antenna are bestinterrogated with orthogonally polarized antennas. The distinctivebenefits of the proposed polarization tuner are simple structure, costsaving, simultaneous control of polarization axial ratios and tileangles, and the control of orthogonality of the two polarizations.

FIG. 4B illustrates a diagram of a polarization adjustable antenna withmatching circuits. In one embodiment, a single-feed polarizationadjustable antenna 450 can be developed for the mono-static RFIDreaders. The single-feed polarization adjustable antenna 450 contains adual-polarized patch 461. The dual-polarized patch 461 has at least oneradiating element capable of supporting orthogonal resonant modes, afirst RVA 453, a second RVA 454, and a power combiner 452. In oneembodiment, the power combiner 452 comprises at least one switchablematching circuit to match the impedances of the combiner 452. The powercombiner 452 receives input signal 451. The first RVA 453 feeds signals456 to Co-POL 459. The second RVA 454 feeds signals 455 to X-POL 460.The operation of the first RVA 453 and the second RVA 454 is illustratedin the following examples. If the patch 461 provides signals fromvertical polarized CO-POL 459 and horizontal polarized X-POL 460, the 45degree linear polarized reception would require equal amount of signalfrom the CO-POL 459 and X-POL 460 radiating elements, and the horizontalpolarized reception would require all signal power from the X-POL 460radiating elements. The CO-POL RVA should be set to theno-loss-phase-shifting mode for the former case and to theno-loss-all-reflection mode for the latter case. The input impedance forthe former is 50 ohm, and the input impedance could be zero, infinity,j50 ohm, −j0.02 or any other point on the unity reflection coefficientcircle on the Smith chart. A circularly polarized reception may beobtained by the quadrature combing of signals from CO-POL 459 and X-POL460.

FIG. 4C illustrates a diagram of a stubbed matching circuit for a 25/50ohm variable load. In one embodiment, the impedance matching circuit 470developed to match RVA for the single-feed tunable polarization antenna.This impedance matching circuit 470 comprises a three-connection Tee473, a 50-ohm one-tenth-wavelength transmission line 474 coupled to thethree-connection Tee 473, a matching capacitor 476 and a switch 475. Thethree connection Tee 473 is also coupled to Co-POL RVA 471 and X-POL RVA472. The switch 475 is coupled to the 50-ohm one-tenth-wavelengthtransmission line 474 in one end, and is coupled to the matchingcapacitor 476 in the other end. The match capacitor 476 is coupled to aterminal 477.

The targeted input impedance at the three-connection Tee is 50 ohms and25 ohms. When equal percentage combination of CO-POL and X-POL arerequired, 25-ohm equivalent impedance is presented at the Tee. Whentotal transmission is required for CO-POL, the X-POL RVA is set to atotal reflection state with infinity impedance. The equivalent impedanceof the Tee becomes 50 ohm.

FIG. 4D illustrates a diagram of a branch selective matching circuit fora 25/50 ohm variable load. In one embodiment, the impedance matchingcircuit 480 contains a three connection Tee 483, a first single poledouble throw (SPDT) switche 484, a second single pole double throw(SPDT) switch 487, an arbitrary length 50-ohm transmission line 485, anda quarter-wavelength 35-ohm transmission line 486. The three connectionTee 483 is also coupled to Co-POL RVA 481 and X-POL RVA 482. The twotargeted impedances for matching at the Tee 483 is also 50 ohms and 25ohms. The 35-ohm transmission line 486 may be selected by the twoswitches 484 and 487 to match the 25-ohm equivalent impedance to 50ohms. The output passed to transceiver signal 488 through the switch487. The TX leakage of a mono-static reader is heavily affected by thereader and antenna impedance mismatch. These two types of power combinerand matching provide matched antenna impedance to mono-static readerwithout any termination loss.

Amplified back-scattering modulated RFID tag systems have a higher tagresponse signal to boost the reader range. FIG. 5A illustrates a diagramof a bi-static RFID tag with amplifier and modulator. In one embodiment,a semi-active RFID tag 500 is develop for longer reader range thanpassive antennas with a back-scattering modulation efficiency greaterthan unity. This tag 500 contains a receiving antenna 501, an RFamplifier 502, an envelope detector 503, an spst switch 508, atransmitting antenna 509, a memory unit 511, a micro-processor 510, anda battery 512. In one embodiment, during the TX mode, external R2Tsignals are received by the CO-POL antenna 501 and directed to the RFamplifier 502. The amplified signal is demodulated by an envelopedetector 503, which is made of a diode 504 and a low pass RC filter thatcomprises a resistor 506, a capacitor 505 and a terminal 507. Thedemodulated R2T signal is processed by the microprocessor 510 toinitiate further operations such as reading or writing memory to thememory unit 511, controlling the modulator 508, or communicating to aremote host 516, a keypad 517 or a display 518. During the transmittingmode, the modulation of T2R signal is commanded by the micro-processor510 through the spst switch. The modulated signal is then sent to atransmitting antenna 509 to broadcast the T2R message. A battery 512 isused as the primary power source to operate the processor 510 and theamplifier 502. A remote host 516 may be connected to the tag, and thetag may be alternatively power by the interface connection.

In some embodiment, the receiving and transmitting antennas haveorthogonal polarizations. The RX antenna polarization here is designatedas CO-POL, and TX antenna polarization is designated as X-POL. A lowcoupling ratio is expected between the two antennas since the fieldorientations are expected to be orthogonal and not correlated. As theRFID tags are usually attached to objects, the mutual coupling to theobject will degrade the isolation between the two antennas. If the loopgain reach unity and amplifier start to oscillate, full RF power isgenerated and the envelop detector 503 is configured to detect the stateof oscillation. To suppress the oscillation, the gain of amplifier 502can be reduced by the micro-processor 510 upon the detection of strongRF signal at the RF power detector 503. If an abnormal amplitude isdetector at the envelop detector 503, the micro-processor 510 couldreduce the amplifier gain. This automatic gain adjustment mechanism notonly suppresses internal oscillation, but also keeps the readeroperating in the linear mode when a strong reader signal arrived at thetag RX antenna. Exiting tag may utilize a circulator to duplex theantenna for transmission and reception. That type of tag has a higherinstability since the reflection coefficient of an antenna is generallyhigher than the coupling between a pair of cross-polarize antennas whendetuned.

FIG. 5B illustrates a diagram of a Bi-static RFID tag with amplifiermodulator and leakage canceller for stability control and modulation. Inone embodiment, a second semi-active RFID tag 520 comprises a CO-POLreceiving antenna 526, a RVA 521, a second directional coupler 528, aX-POL antenna 531, an RF amplifier 522, an spst switch 523, a powersupply 524, an RF detector 529.

In one embodiment, In this RFID tag 520, a portion of receiving R2Tsignal is directed to the RVA 521, and the subsequent phase andamplitude adjusted R2T signal is added back to the amplified T2R signalbefore RF detection. One method to modulate the R2T signal is by eithersupply or not supply DC power 524 to the amplifier through the SPSTswitch 523. Because a portion of transmit signal is coupled back to thereceiving antenna to cancel the coupling between two antennas. Thefollowing RF detector 529 produces an error signal to indicate thequality of cancellation for the micro-processor. The antenna stabilityis improved since the leakage-induced loop gain is reduced. Therefore,the gain of amplifier 522 does not need to be sacrificed for stability.

An amplifier often allows different forwarding gain at different supplyvoltages. By varying the forward gain of amplifier 522, the transmittingpower is then amplitude modulated. Therefore, this tag may be configuredto produce back-scattering modulated signal of narrower bandwidth thanon-off keying modulated signals at the same data rate. Another method tomodulate the R2T signal is to apply phase modulation or frequencymodulation by controlling the phase shift of the RVA. Therefore, there-transmitted signal may be phase modulated or frequency modulated.Additionally, an switch 523 is connected to the bias of amplifier totoggle between two gain stages to perform deep On/off modulation.

A portion of transmitter characteristic may be transferred to a remotehost of the tags 520. Each transmitter may have its own identificationnumber, and this number may be transferred to the tag host along withthe time of interrogation. Therefore, additional services may beenabled. One application is to have the tags installed at a gate, andupon the reading of tag, the reader identification may be sent to thehost of tag to operate the gate, or to track the path of the reader.

The circuits shown in FIG. 5A and FIG. 5B enables complex modulation ofthe received RF signal. By adjusting the vector attenuator 521, it ispossible to change the modulus of the reflection coefficient, thereforeto amplitude modulate the back-scattered signal at the receivingantenna. The transmission coefficient may also be adjusted in the samefashion, therefore to amplitude modulate the re-transmitting signal forthe transmitting antenna. The vector phase shifter 521 may be adjustedquadraturely to apply phase shift to the reflection coefficient for thereceiving antenna. Therefore, phase modulated back-scattered signal maybe obtained at the receiving antenna. Frequency modulated back-scatteredsignal may also be obtained by continuously modulating the RVA with twosinusoidal signals, where one sinusoidal signal is a phase shiftedversion of the other signal. In the same fashion, the re-transmittingsignal at the transmitting antenna may also be phase shifted orfrequency modulated, using a different set of coefficients as the RVAcontrol signals.

FIG. 5C illustrates a diagram of a Bi-static RFID tag with polarizationcontrol to provide a means to tune the tag polarization. Thepolarization In one embodiment, a bi-static RFID tag comprises a dualpolarization tuner 540, a CO-POL Receiving Antenna 543 coupled to thetuner 540, a X-POL Receiving Antenna 544 coupled to the tuner 540, anamplifier 542 coupled to the tuner 540, a RF detector 541, a modulationcontrol 547 by a switch 548, a power supply 546. One method to constructthe polarization tuner 540 is previously shown in FIG. 4A. There areexternal polarization control 549 feeds into the dual polarization tuner540. In another embodiment, the external polarization control 549 iscontrolled by a micro-processor. In one embodiment, the RF detector 541has a baseband output signal 545. In some embodiments, the polarizationcontrol can enhance tag stability by reducing antenna coupling, andimprove reception sensitivity by matching the tag antenna polarizationto the multi-path combined wave.

The circuits 500, 520 and 550 may also be used to construct a radiorelay to improve RFID read range. Similar to the disclosed semi-activeRFID tag, the function of relay is to re-transmit the received RF waveat a higher amplitude. Therefore, FIG. 5A, FIG. 5B or FIG. 5C may be maybe understood as the representative of a relay. A relay is placedbetween a RFID reader and a RFID tag. In one configuration, the relaymay be used to enhance the reader to tag channel, so the relay RXantenna 501 may be pointed towards a reader, and relay TX antenna 509may be pointed towards a tag. In another configuration, the relay may beused to enhance the tag to reader channel, so the relay RX antenna 501may be pointed toward a tag, and relay TX antenna 509 may be pointedtowards a reader. A relay does not re-modulate received RF waves, so theon/off modulator switches 508, 523 and 548 may be removed from thecircuits. Because of the internal amplifier 502 and antenna couplings,the relay is subject to oscillation under circumstances. Stabilitymonitoring is performed by the power detector 503, 529 or 571. Stabilitycontrol is performed by the adjusting the RVA 521 or 571. Tuning of theantenna polarization by adjusting the polarization tuner 540 is anotherbenefit since the same multipath effect also applies to the reader torelay communication link and relay to tag communication link.

Another method to construct the variable gain amplifier 502 is shown inFIG. 5D. This variable gain amplifier 570 comprises a RVA 571, and anamplifier 572. The RVA 571 may be connected to the either the input orthe output of the amplifier 572. This variable amplifier is functionallyequivalent of a vector modulator, or vector amplifier. This variableamplifier may have a variable gain, and a variable phase.

1. A leakage cancellation circuit, comprising: a mixer, wherein themixer is configured to down-mix a first copy of a receiver signalcontaining transmission leakage with a first copy of a transmittersignal and generate an output; a first low pass filter coupled to themixer, wherein the first low pass filter is configured to filter theoutput and generate a first reference voltage; a second low pass filtercoupled to the mixer, wherein the second low pass filter is configuredto filter the output and generate a second reference voltage; a linearcombiner coupled to the first low pass filter and the second low passfilter, wherein the linear combiner is configured to generate a firstcontrol voltage and a second control voltage, wherein the first controlvoltage is related to a first linear combination of the first referencevoltage, the second reference voltage and a first coefficient, whereinthe second control voltage is related to a second linear combination ofthe first reference voltage, the second reference voltage and a secondcoefficient; and another mixer coupled to the linear combiner, whereinthe another mixer is configured to up-mix the first control voltage, thesecond control voltage and a second copy of the transmitter signal,wherein the another mixer is configure to add output of the anothermixer back to signal for receiving and cancel transmission leakage. 2.The leakage cancellation circuit of claim 1, wherein the liner combineris configured to produce the first control voltage (CVI) and the secondcontrol voltage (CVQ) from the first reference voltage (EVI) and thesecond reference voltage (EVQ), according to a following equation:CVI=a*EVI+b*EVQ+c, CVQ=d*EVI+e*EVQ+f, wherein a, b, c, and d arecoefficients related to dynamic leakages, wherein c and f arecoefficients related to stationary leakages.
 3. The leakage cancellationcircuit of claim 1, wherein a radio frequency power detector isconfigured to monitor quality of the leakage cancellation circuit.
 4. Aleakage cancellation circuit, comprising: a mixer, configured to mix areceiver signal with either a copy of a transmitter signal or a localoscillator signal and generate at least two outputs; a first low passfilter coupled to the mixer, wherein the first low pass filter isconfigured to filter the at least two outputs and generates a firstreference voltage; a second low pass filter coupled to the mixer,wherein the second low pass filter is configured to filter the at leasttwo outputs and generate a second reference voltage; at least two analogto digital converters coupled to the first low pass filter and thesecond low pass filter, wherein the at least two analog to digitalconverters are configured to digitize the first reference voltage andthe second reference voltage; a digital signal processor unit coupled tothe at least two analog to digital converters; at least two digital toanalog converters coupled to the digital signal processor unit, whereinthe digital signal processor unit and the at least two digital to analogconverters are configured to produce two control signals; a vectorattenuator coupled to the at least two digital to analog converters,wherein the vector attenuator is controlled by the two control signalsto produce a negative copy of transmission leakage, wherein the negativecopy of transmission leakage is combined back to the receiving signal tocancel the transmission leakage; an amplifier coupled to the leakagecombiner and the mixer, wherein the amplifier is configured to reducethe noise figure of a down-converter; and at least two high pass filtersor bandpass filters coupled to the mixer, wherein the at least two highpass filters or bandpass filters are configured to reduce thetransmitter leakage.
 5. The leakage cancellation circuit of claim 4,wherein the two control signals for the vector attenuator are linearcombinations of the two reference voltages.
 6. The leakage cancellationcircuit of claim 4, wherein operation of the linear combinations isprocessed by the digital signal processor.
 7. A radio relay device thatre-transmits received radio-frequency wave at a higher amplitude,comprising: a receiving antenna, wherein the receiving antenna isconfigured to receive external signals; an amplifier coupled to thereceiving antenna, wherein the amplifier is configured to amplify theexternal signals and generate amplified signals; a transmitting antennacoupled to the amplifier, wherein the transmitting antenna is configuredto re-transmit the amplified signal; a circuit to identify operatingcondition of the amplifier, comprising an envelope detector coupled tothe amplifier, wherein the envelop detector is configured to detectpower level of the amplified signals and generate another signal; aprocessor coupled to the envelop detector, wherein the processor isconfigured to compare the another signal to a pre-determined levelsignal to initiate activities including but not limiting to reading orwriting memory, accessing input or output pins, or adjusting connectedcomponents; and a memory unit coupled to the processor, wherein thememory unit is configured to store information.
 8. The radio relaydevice of claim 7, wherein the amplifier is configured to have variablegain.
 9. The radio relay device of claim 7, wherein the amplifier isconfigured to have variable phase.
 10. The radio relay device of claim7, wherein the polarizations of the receiving antenna or transmitantenna is tunable.
 11. A modulator of a radio frequency signal,comprising A first antenna; a vector attenuator coupled to the firstantenna; and a second antenna coupled to the vector attenuator, whereinthe vector attenuator is configured to modulate received signal from thefirst antenna and generate another signal, wherein the second antenna isconfigured to retransmit the another signal.
 12. The modulator of theradio frequency signal of claim 11, wherein the another signal isamplitude-modulated, by applying at least one control signals to thevector attenuator to affect insertion loss of the vector attenuator. 13.The modulator of the radio frequency signal of claim 11, wherein theanother signal is phase-modulated or frequency-modulated, by applyingtwo control signals at the vector attenuator, to change phase delay ofthe vector attenuator.